Long Term Evolution (LTE)/LTE-Advanced employs MIMO technologies to increase, for example, the system capacity and the cell edge user throughput. Also, Long Term Evolution (LTE)/LTE-Advanced employs heterogeneous network technologies that make it possible to mix different types of base stations (e.g., macro cells and small cells) while reducing the inter-cell interference and achieving high-quality communications.
Particularly, it is expected to use high-frequency bands for small cells in a heterogeneous network. Here, because propagation loss increases in a high-frequency band, it is being considered to compensate for the propagation loss by using massive MIMO where beams with narrow beam widths are formed.
The massive MIMO is a large-scale MIMO system where a large number of antennas (e.g., 100 elements) are used. The massive MIMO makes it possible to concentrate the electric-field intensity in a narrow area and thereby makes it possible to reduce the interference between users.
Also, for a heterogeneous network, it is being considered to perform beam forming using multiple antennas both for uplink and downlink to compensate for propagation loss in a high-frequency band.